1. Field of the Invention
The present invention relates to a process for producing an organopolysiloxane with high polymerization degree, and more particularly to a process for producing an organopolysiloxane with high polymerization degree endblocked with a triorganosilyl group while preventing effectively the formation of low-molecular-weight siloxanes with low polymerization degree.
2. Description of the Prior Art
Silicone rubbers, with their excellent weatherability, electrical properties, low-compression-set properties, heat resistance, cold resistance and the like, are widely used in electronic apparatus, automobile, construction, medical, food and other diverse fields. The applications for which silicone rubbers are in use include, for example, rubber contacts for use in rubber contact keys in remote controllers, typewriters, word-processors, computer network terminals, musical instruments, etc.; gaskets for construction; diverse rolls such as copying-machine rollers, developing rollers, transfer rollers, electrification rolls, paper feed rollers, etc.; vibration-proofing rubber for audio devices, etc.; packings or gaskets for compact disks used in computer systems, and so forth.
There is an increasingly greater demand for silicone rubbers, and development of silicone rubber with excellent characteristics is desired.
Among others, reduction in the amount of low-molecular-weight siloxanes contained in a high-polymerization-degree organopolysiloxane is keenly demanded. In a contact rubber used for contact keys, for example, low-molecular siloxanes may cause contact failure. In copying-machine roll or roller materials, low-molecular siloxanes may cause the so-called "off-set" phenomenon in which a toner is transferred onto the roll through the low-molecular siloxane. It is also known that in gaskets for construction use or the like, one of the major contaminants is the low-molecular siloxanes which remain in the organopolysiloxane constituting the gasket or the like.
In view of the above, various methods have been investigated for reducing the amount of low-molecular siloxanes contained in an organopolysiloxane having a high polymerization degree.
As a method of reducing the low-molecular-weight siloxane content, is known a method in which a high-polymerization-degree organopolysiloxane (which hereinafter may be referred to simply as "siloxane gum") is subjected to stripping at a high temperature of 100.degree. to 300.degree. C. under a reduced pressure. Because of the high viscosity of the siloxane gum, however, tremendous energy and time is needed to remove the low-molecular siloxanes assuredly. This method is thus limited in its practical use. Therefore, a method for obtaining a siloxane gum in which the amount of low-molecular siloxanes is extremely small has been requested.
A method of producing a siloxane gum is disclosed, for example, in U.S. Pat. No. 2,634,252 in which a siloxane gum is obtained through an equilibration reaction using potassium hydroxide, potassium silanolate, lithium silanolate, tetraalkylammonium hydroxide or the like as a catalyst. The siloxane gum obtained using such a catalyst is endblocked with silanol, and its terminal ends can be easily treated. According to this method, however, the resulting siloxane gum contains a considerable amount (about 5 to 10% by weight) of cyclic low-molecular siloxanes, removal of which requires huge costs.
Also, methods are known for producing a siloxane gum without formation of low-molecular-weight siloxanes. For example, a method is known in which a silanol is subjected to dehydration condensation using an acidic compound having a pK.sub.a of 3 or less at 20.degree. C. (European Patent Publication 0431980 corresponding to Japanese Pre-examination Patent Publication (KOKAI) No. 3-179027). This method is not yet satisfactory in preventing effectively the formation of low-molecular siloxanes.